Buzan explained that water salinities (salt content) rise as rainfall is reduced and human water use increases and these factors in combination reduce freshwater inflows to coastal estuaries where rivers meet the sea. High temperatures and salinities reduce the amount of oxygen water can hold. Smaller tidal changes and the absence of typical strong winds further reduce the amount of oxygen normally added to water by wave action and physical mixing. Natural physical mixing of the water is even more reduced in man-made residential boat canals, where some kills have been reported this month.

Recent fish kills this month in Chocolate Bayou, the old mouth of the Colorado River and several small incidents in the Corpus Christi area typify these low oxygen fish kills.

Biologists are closely watching the situation in the Colorado River near its mouth. This is an area where 60 million small fish died in a similar low dissolved oxygen incident in 1995. Early this month, fish kill numbers here were in the thousands, but biologists are concerned that the situation could get worse.

Fishermen and beachgoers often observe low oxygen conditions early in the morning when they see fish swimming slowly at the surface of the water.

“Fish can appear to be ‘gasping’ for air as they try to get oxygen in the surface of the water,” Buzan said. “During severe low oxygen conditions, the fish ‘gasping’ behavior can be seen throughout the day.

The types of fish usually killed in high numbers by these incidents include menhaden, a forage species that provides food for larger fish. Menhaden are also commercially harvested for pet food, so they are important ecologically and commercially.

Buzan said Menhaden can die in high numbers, but they are the type of fish that tends to reproduce in the millions, so their population can bounce back in a year. Other types of fish are also affected, but biologists rarely see large kills of highly mobile game fish such as red drum, speckled trout and flounder because these fish are less likely to be trapped in areas of low oxygen.

TPWD biologists continue to investigate these fish kills and keep track of what is happening. Biologists are asking the public to continue to contact the department's 24-hour hotline at (512) 389-4848 if they see fish, shrimp or crabs that are dead or appear to be stressed.

TH 2005-08-08

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